388 PROCEEDINGS OP SECTION D. 



transporting power is increased sixty-four times. It is necessary 

 to state that the general applicability of this principle is disputed 

 by some authorities ; but, on the other hand, its correctness in 

 theory is established, and its practical accuracy, in relation to some 

 cases at least, has been verified by careful experiments. 



These considerations all tend to show that while the position of 

 the source of a river is ever varying, and the destination of its 

 waters is uncertain, so also the condition of the channel is one of 

 perpetual change. As the products of erosion in the higher parts 

 of the catchment area accumulate in the valleys and on the low- 

 lying lands, the course of the river lies in an ever-increasing 

 extent through deposits of its own creation. In the portions of a 

 river channel situated among hills, no great change of position 

 can be looked for ; the effects of erosion being confined chiefly to 

 the bed of the channel. But in the alluvial deposits the case is 

 very different. Here a trifling obstruction is sufficient to divert 

 the current against one or other of the banks ; and the bank so 

 acted on is eroded, whilst at the same time the resistance which 

 it offers has the efiect of directing the force of the current against 

 the opposite bank at a point further down. It is evident that 

 after a bend has begun to form in this manner, the natural 

 tendency is to increase the abruptness of the bend till at last 

 the neck is cut through and an island is formed at one side of the 

 river, and bounded by a horse-shoe-shaped lagoon. This process of 

 erosion of the bank results in constant changes of the course of 

 a river through alluvial deposits ; but much more important 

 changes sometimes occur through the alteration of the lelative 

 levels of the river and the land adjoining, caused by the deposit 

 of silt. When a river channel passes through alluvial deposits 

 where the rate of fall is slight and the velocity materially 

 diminished, the silt carried in suspension is deposited and the 

 bed of the river raised. The natural eft'ect of this is to cause 

 overflows in times of flood ; and just as the silt is deposited in the 

 channel on account of the diminished velocity when the river is 

 low, so silt is deposited on, and near the river bank on account of 

 the check which the velocity of the overflowing waters receive 

 through their sudden spreading out over comparatively level lands. 

 This silt takes approximately the form of a wedge with its thick 

 end resting on the river bank. It is apparent that the effect of 

 this process is to raise the bed and banks of the river above the 

 adjacent land. -When this stage is reached an important change 

 in the course of the river is only a question of time. Erosion of 

 either bank would lead to a complete change of channel, as the 

 river after breaking through would seek the lowest ground and 

 follow it. 



I have endeavoured in the foregoing to describe in general 

 terms the leading characteristics of rivers, the influences which 

 affect their discharge, and the processes by which their conditions 



